More and more computer devices are equipped with touch screens. In general, touch screens are classified into capacitive type, resistive type, acoustic type, and optical type. For example, Acer™ Gateway ZX6810 from Acer company and iPhone™ from Apple company adopt the capacitive-type touch screens, and T91 Tablet from ASUS company, 1420P-232G32n from Acer company, and N800 Internet Tablet from Nokia company adopt the resistive type touch screens.
In response to the usage of touch screen, iPhone™ provides a virtual keyboard, wherein different texts such as capital letters and lower case letters can be inputted according to the longer or shorter touch duration. Moreover, in the article titled “Pressure-based text entry for mobile devices” by Stephen A. Brewster and Michael Hughes, Proceedings of the 11 th International Conference on Human-Computer Interaction with Mobile Devices and Services, Sep. 15-18, 2009, Bonn, Germany, it also suggested that virtual keyboard adopted in N800 Internet Table can input capital or lower case letters by different touch pressures.